System for detecting a signal in the presence of noise



Jan. 31, 1961 E. LAKATOS 2,970,304

SYSTEM FOR DETECTING A SIGNAL IN THE PRESENCE OF NOISE Filed Feb. 12,1958 2 Sheets-$heet 2 \MH'ER. FREQUENCY D\S(.RIM|NATOR EMORy ZAKA TosINVENTOR.

Bi gi w A 7TORNE Y8 United States Patent SYSTEM FOR DETECTING A SIGNALIN THE PRESENCE OF NOISE Emory Lakatos, Los Angeles, Calif., assignor,by mesne assignments, to Thompson Ramo Vyoolorldge Inc., Cleveland,Ohio, a corporation of Ohio Filed Feb. 12, 1958, Ser. No. 714,768

18 Claims. (Cl. 340-248) This invention relates to a system fordetecting the presence of an information signal component in a compositeinformation and noise signal. In particular, the invention relates to asystem adapted to detect the presence or absence of an informationsignal having one frequency characteristic in a noise environment havinga frequency characteristic different from that of the informationsignal. Whie not limited thereto, the invention is herein described withreference to the detection of the presence or absence of a substantiallyunifrequency sine wave information signal component in an incoming radiofrequency signal containing a noise signal component with a widerfrequency band width than that of the information signal component to bedetected. The invention proves especially advantageous in the detectionof the presence or absence of the information signal on a go, no gobasis when the signal to noise ratio is as low as 1 or less.

According to the invention use is made of the phenomenon wherein, if asubstantially unifrequency information signal (for examp'e a sine wavesignal) and a noise signal are passed through certain non-lineardevices, the information signal will modulate the noise so that thenoise power at the output of the device will be less in the presence ofthe information signal than in the absence of it. This phenomenon isused, according to the invention, in providing information as to thepresence of the information signal; an appropriate utilization device,such as alight or alarm signa is connected to be actuated by thedetection of the presence of the information signal, or more properly,by the reduction in the power content of the noise in the incomingsignal sample.

In the detecting system of the invention a frequency selective ordiscriminator circuit, of the type capable of converting frequencychanges into marked voltage changes, is used to separate an input signalhaving noise and information components into a transformed signal havingan alternating current portion and a substantially direct currentportion. The alternating current portion, representative of the noisecomponent of the input signal, is fed into a power measuring device,such as a fastacting thermocouple, to give an indication of the powercontent of the alternating current noise-representing sig nal. Since thepower content of the noise-representing signal is larger in the absenceof an incoming information signal component than in the presence of sucha com ponent, a low power content indicates the presence of aninformation signal. The power content indication signal from the powermeasuring device is fed to a comparing circuit where the magnitude ofthis signal is compared with a reference; a power-representing magnitudelarger than the reference indicates the absence of an informationsignal, while a magnitude smaller than the reference indicates thepresence of an information signal.

In the drawings, wherein like reference characters refer to like parts:

Figure 1 is a block diagram illustrating a signal detecting systemembodying the invention;

2,970,304 Patented Jan. 31, 1961 Figure 2 is a partially schematicdiagram illustrating the operation of a system of the type illustratedin Figure 1;

Figure 3 is a schematic illustration of an alternate form of a portionof a system ilustrated in Figure 2; and

Figure 4 is a graphical illustration of the sensitivity of a detectingsystem embodying the invention.

Environments are known wherein it is desirable to detect, on a go, no gobasis, the presence or absence of an information signal component in anincoming signal having a large noise level. For example, in detectingthe presence of a radar target it is desirable to know when a radarreturn signal is present even though the character of the return signalcannot be deciphered due to a high ambient noise level. Thus, .forexample, in an aircraft radar set designed to indicate the possibilityof a collision with another aircraft, it is desirable to know of thepresence of the other aircraft in the vicinity. Consequently, the merepresence of a radar return signal provides the information desired. Thesystem of the invention enables the foregoing to be realized even whenthe noise level is higher than the level of the information signal. Thesystem of the invention may be used, for examp'e, in a collision warningarrangement of the kind described in a copending application Serial No.587,768, filed May 28, 1956, and assigned to the same assignee as thepresent application.

A block diagram of the system of the invention is illustrated inFigure 1. An incoming signal is fed into a frequency selective ordiscriminating circuit 10, a nonlinear signal transformation circuitthat provides a large change in output voltage magnitude for a smallchange in signal input frequency, for example a change in voltagemagnitude of the order of 15 millivolts for a frequency change of onecycle. Such circuits are well known in the art and may be, for example,one of the known limiterdiscriminator circuits described in Section 7,paragraph 18, pages 585 to 588, Radio Engineers Handbook, by FrederickE. Terman, published in 1943 by the McGraw- Hill Book Company, New York.The output from the frequency discriminator circuit 10 is fed into adirect current blocking circuit 12 where substantially only thealternating current component of the signal is allowed to pass.

The output from the direct current blocking circuit 12 is fed into apower measuring device 14, such as one of the known fast-actingthermocouples, and the signal from the power measuring device,representative of the power level of the alternating current component,is fed into a comparing circuit such as an adder 16.

The adder 16 issued in providing an indication as to whether themeasured power is relatively small, indicating the absence of aninformation signal component in the original, incoming signal, or isrelatively large, indicating the presence of the information signalcomponent. A reference or bias signal, from a bias source 18, is alsofed into the adder circuit 16. The power representing signal, from thepower measuring device 14, and the bias signal, from the bias source 18,are of opposite polarity. The algebraic sum of these two signa s, aresultant signal having the polarity of the larger of the bias or powerrepresenting signals, is fed through a unilateral conduction device 20,such as a rectifier, for passing only a resultant signal when it has thepolarity of a predetermined one or the other of the bias or powerrepresenting signals. For example, the unilateral conduction device 20may be oriented to pass only signa s of the polarity of the bias signal.Under these circumstances a signal will be passed through the unilateralconduction device 20 only when the magnitude of the power representingsignal into the adder 16 is lower than a predetermined value,representng the absence of appreciable noise power in the originalincoming signal (corresponding to the presence of an information signalcomponent in the input signal to the system). The output from theunilateral conduction device 20 is fed into a signal utilization device22, which may for example be an alarm circuit, for indicating thepresence of the information signal component.

The operation of the system of the invention will now be discussed. Asis known, a frequency selective circuit that transforms frequencychanges into voltage changes, such as a frequency discriminator circuitof the type referred to, transforms an input signal frequency changeinto an output signal voltage level change. Thus, when a substantiallyconstant frequency input signal (for example a constant frequency,information component, sine wave signal) is applied to the discriminatorcircuit, this circuit will provide a substantially constant level orsubstantially direct current output signal 2611. However, the noisecomponent accompanying the substantially constant frequency sine wavesignal component will have a random variation in frequency since thenoise signal component has a wider band width than that of thesubstantially unifrequency signal. Thus, when the noise component passesthrough the discriminator circuit the discriminator circuit will providean output signal having voltage variations corresponding to thefrequency variations of the input noise signal. Consequently. the outputfrom the discriminator will have a changing voltage level (forconvenience this changing voltage level will be re ferred to as analternating current although in reality the current may be a unipolaritycurrent having relatively large amplitude variations). This changingvoltage level is representative of the noise component input to thediscriminator. Since the information signal is now representcd by thesubstantially direct current component of the discriminator output whilethe noise component is represented by the alternating current componentof the output, when the output from the discriminator circuit is passedthrough a capacitor 32 the information signal component will be blockedand only the noise level component will pass.

In the absence of an information signal component in the incomingcomposite signal to the discriminator circuit the noise signal componentconstitutes the entire signal input to the circuit and provides a givenalternating current power level output. In the presence of aninformation signal component, however, this information com ponent willmodulate the noise component. Consequently, the noise power level at theoutput of the circuit will be less in the presence of the informationcomponent than in the absence of it. Even in the presence of a weakinformation signal component at the input to the circuit, the powerlevel of the noise component at the output of the circuit is markedlyreduced in comparison to the power level of the noise component in theabsence of the information component. The basic principles underlyingthis phenomenon are stated in mathematical terms in an article entitledStatistical Properties of a Sine Wave Plus Random Noise, by S. 0. Rice,in the Bell System Technical Journal, pages 109 to 157, January 1948, inparticular Figure 8 of this article. As illustrated in Figure 4, aderivation from Figure 8 in the article referred to, the power level ofthe noise component of the signal drops to about 60 percent of itsoriginal value when the signal to noise ratio increases from to .5. Thismeans that the presence of an information signal may be detected evenwhen there is twice as much noise present as there is signal. Of course,when the signal to noise ratio is higher the noise power drops evenlower. Thus, for example, with a signal to noise ratio of 2 to 1 thenoise power drops to 10 percent of that present in the absence of anyinformation signal.

Thus it is seen that the power level of the signal passing through theblocking capacitor 32 will be markedly less during time intervals whenan information signal component is received by the system than duringtime intervals when no information signal component is present. Thedifference in the blocking capacitor output signal when an informationsignal component is absent, relative to the output when such a signal ispresent, depends upon the signal to noise ratio of the over-allcomposite signal input to the limiter-discriminator circuit. When thesignal to noise ratio is of the order of 1 to 1 the reduction in noisecomponent power is of the order of 65 percent of its value in theabsence of any information signal, as illustrated in Figure 4.

The foregoing system will now be described in greater detail inconnection with Figure 2. The incoming composite information and noisesignal is graphically illustrated in Figure 2 with a depiction of anoise level component, graph line 24, and an information signalcomponent, graph line 26. The coordinates of the graph line depictsignal amplitude over time. For convenience of explanation let ussuppose that during time periods B no information signal component ispresent, and then during time periods A an information signal componentis present. The incoming composite signal, graph lines 24 and 26, ispassed through a band pass filter 28 for removing the portions of thenoise signal component that are appreciably above and below thefrequency of the information signal component, the filter having a wideenough pass band to pass substantially the entire information signalcomponent, a substantially sine wave signal. The noise and informationcomponents in the signal emerging from the band pass filter 28 areillustrated by graph lines 24a and 26a. The output from the band passfilter 28 is passed through a limiter-frequency discriminator circuit 30of the type referred to, having frequency discriminating action at thefrequency of the information component. In the discriminator circuit 30the relatively uniform information signal component 26a is transformedinto a unipolar or substantially direct current signal portion 26b andthe noise component 24a is transformed into an alternating currentsignal portion 24b. For the reasons described in the article referred tothe noise representing alternating current portion 24b has a relativelylarge magnitude during the time B that an information signal componentis not present and a relatively small magnitude during the time A thatan information signal component is present. The com-posite output signal24b and 26b from the limiter-frequency discriminator circuit 30 is fedinto a direct current blocking circuit, which may comprise a capacitor32, for substantially removing the direct current component 26c from thecomposite signal and passing substantially only the alternating currentcomponent 24c. Thus, the information signal component, represented bythe substantially direct current level, is not present while the noisesignal component, represented by the alternating current level, has amarkedly reduced magnitude during the time period A during which theinformation signal component was present. The alternating currentcomponent has a small transient, t, produced by the sudden presentationof the information signal component to the capacitor 32. However, aswill be shown, the presence of this transient has substantially noeffect on the operation of the system.

The output from the blocking capacitor 32 is fed into a power measuringdevice such as, for example, a thermocouple 34 of a known type having arelatively fast response time. The thermocouple is preferably of avariety having a response time of appreciably less than one second, forexample from of the order of A to of the order of ,4 of a second. Sincesuch thermocouples are known in the art they will not be furtherdescribed. A low pass filter 36, for example an R-C filter comprising aresistor 38 and capacitor 40, is preferably connected between theblocking capacitor 32 and the thermocouple 34 for blocking alternatingcurrent having a frequency higher than that which the thermocouple iscapable of responding to. This filter 36 substantially eliminatesharmonics and aids in the restriction of power into the thermocouple 34to only the desired signals. Since the magnitude of the alternatingcurrent fluctuations during the absence of an information signal isgreater than the magnitude of the fluctuations during the presence ofthe information signal, the power measured by the thermocouple,represented by a unipolar power representing signal 24d, will be greaterduring the time period corresponding to the absence of an informationsignal than that during the presence of an information signal. Theaforementioned transient I will be present as a small power pip t. Thepower representing output signal from the thermocouple 34 may haveeither a positive or negative unipolarity. In the embodiment shown byway of example the power representing signal is chosen to have apositive polarity.

This power representing signal 24a is fed into one of two inputs to acomparing or adder circuit 42. A bias signal 242, from a bias signalsource 44, is fed into the other input to the adder circuit 42. The biassignal Me has a polarity opposite that of the power representing signal24d from the thermocouple 34 and an absolute magnitude intermediate thatbetween the lowest amplitude g of the no-informationsignal representingportion of signal 24d and the amplitude 11 of the signal informationrepresenting portion of signal 24d. The absolute magnitudes of the twosignals 24d and 24e are compared by being algebraically added by theadder circuit 42, and the sum of these signals is fed to a unipolarityconducting device such as a rectifier 46 oriented to pass only one ofthe larger of the signals representing the bias signal 24:: and thepower representing signal 24d. In the embodiment shown by Way of examplethe rectifier 46 is oriented to pass only negative signals. Thus, onlythe bias signal 24c will be passed, and then only during such periods oftime that the absolute magnitude of the bias signal 24c is larger thanthat of the power representing signal 24d. The foregoing condition ispresent only during the time interval A. Consequent, an output from therectifier 46, represented by wave form 24) is pres out only during thetime period corresponding to the time during which an information signalwas present in the original incoming signal.

The rectifier output signal 24 is fed through a go, no go indicatingdevice, such as a relay 48, for controlling an appropriate alarm circuit50. The alarm circuit is here represented by a light bulb 52 connectedto be energized by a battery 54. Since the power pip 2" occurs for onlya small fraction of the time A during which an information signalcomponent is present, the presence of this pip will not interfere withthe indication of the presence of the information component. Thus, inthe circuit illustrated the lamp 52 will be illuminated during timeintervals corresponding to the intervals A during which an informationsignal was present in the original incoming signal. Consequently, as hasbeen explained above in connection with Figure 4, even a change insignal to noise ratio as small as from 0 to l to .5 to 1 will bedetected by the circuit. The detection level, the sensitivity of thecircuit of Figure 2, is determined by the absolute magnitude of the biassignal level 24:: relative to the absolute value of the powerrepresenting signal level 24a. The greater the magnitude of the biassignal level relative to-signal level g (the bias signal level magnitudealways being less than that of the signal g) the higher the sensitivityof the circuit to the presence of an information signal. However, as isrealized, an increased sensitivity of the circuit to an informationsignal component is accompanied by a corresponding reduction inselectively of the circuit to false alarms, that is, to random noisethatmay happen to look to the systern like an information signal. Thus,if a larger number of false alarms can be tolerated, the circuit may bemade sensitive to signal to noise ratios as small as a very smallfraction of l.

While the invention has been described with respect to circuit elementsof a particular type for convenience of explanation, it will beappreciated that equivalent circuits may instead be used. For example,while the power measuring device of the invention has been illustratedin Figure 2 as being a thermocouple 34, it may instead be a differentpower measuring device. Thus, as illustrated in Figure 3, the powermeasuring device may instead be an integrating network including arectifier 56 and an R-C circuit made up of a resistor 58 and capacitor60. Similarly, while the added 42 is illustrated for convenience ascomprising two resistors 62 and 64 with an output tap from a centerpoint between the re sistors, the adder may instead take a differentform, for example the form of an adder circuit capable of providing anamplified output. Since such adder circuits are well known in the artthey will not be described here.

Since the unifrequency information signal 26 provides the same reductionin measured noise power regardless of the frequency of the informationsignal, provided this frequency is within the limits of the non-linearresponse characteristic of the discriminator circuit 30, the systemproves especially advantageous in detecting the presence of a returnsignal in a Doppler radar set, where the substantially unifrequencyreturn signal is characterized by a very slow change in frequency. Aslong as the frequency of this return signal lies within the frequencyoperating limits of the discriminator circuit 30, the only effect of theslow frequency change will be a corresponding slow change in themagnitude of the direct current output from the discriminator circuit.This slow change in direct current magnitude with change in frequency isillustrated by the slow rising characteristic of the wave formrepresenting the direct current signal portion 26b during time intervalA. Since, as has been explained, this direct current output is not usedin the system of the invention, the change in magnitude of the directcurrent output does not affect the signal detecting ability of thesystem.

From the foregoing it is seen that the improved information signaldetecting system of the invention provides an indication of the presenceof an information signal even when the noise level accompanying theinformation signal is greater than that of the signal level, and evenwhen, as is the case in a Doppler radar return signal, the frequency ofthe information signal is subject to change.

What is claimed is:

1. A detecting system, comprising: frequency selective means adapted toreceive an input signal having noise and information components andconstructed to convert said input signal into a transformed signalhaving an alterating current component, representative of the noisecomponent of said input signal, and a substantially direct currentcomponent representatives of said information component; blocking meansconnected to said frequency selective means for receiving saidtransformed signal therefrom and adapted to remove said substantiallydirect current component from said transformed signal and to pass saidalternating current component which is varied an amount functionallyrelated to the removed information component; power measuring meanscoupled to said blocking means for receiving said alternating currentcomponent therefrom and constructed to convert said alternating currentcomponent. into a signal representative of the power in said alternatingcurrent component; a bias voltage source; and comparing means connectedto receive a bias voltage from said voltage source and to receive saidpower representing signal from said power measuring means andconstructed to provide an output signal representative of the larger ofsaid bias and power signal levels.

2. The system claimed in claim 1 wherein said frequency selective meanscomprises a limiter-frequency discriminator circuit.

3. In a system for detecting the presence of a Doppler radar informationsignal, lying in a predetermined frequency band, in a noise environment;and wherein the signal to noise ratio is of the order of l to l or less:the combination of a limiter-discriminator circuit having a non-lineardiscriminator response characteristic over said frequency band andadapted to receive an input signal having an information componentrepresentative of said radar information signal and a noise component,and constructed to convert said input signal into a transformed signalhaving an alternating current component, representative of the noisecomponent of said input signal, and a substantially direct currentcomponent representative of said information component; blocking meansconnected to said limiter-discriminator circuit for receiving saidtransformed signal therefrom and adapted to remove said substantiallydirect current component from said transformed signal and to pass saidalternating cur rent component; power measuring means coupled to saidblocking means for receiving said alternating current componenttherefrom and constructed to convert said alternating current componentinto a unipolarity signal representative of the power in saidalternating current component; a bias voltage source connected toprovide a direct current bias voltage signal of a polarity opposite thatof said power representing signal; comparing means connected to receivesaid bias voltage signal from said voltage source and to receive saidpower representing signal from said power measuring means, andconstructed to provide an output signal with a polarity the same as thatof the larger of said bias and power signals; a unilateral conductiondevice connected to receive said output signal from said comparing meansand to pass a signal representative of only one of said bias and powersignals when said one signal is larger than the other; and a signalutilization device connected to respond to the passage of said onesignal through said device.

4. A detecting system, comprising: frequency selective signaltransformation means adapted to receive an input signal having noise andinformation components and constructed to convert said input signal intoa transformed signal having an alternating current component,representative of the noise component of said input signal, and asubstantially direct current component; blocking means connected to saidtransformation means for receiving said transformed signal therefrom andadapted to receive said substantially direct current component from saidtransformed signal and to pass said alternating current component; powermeasuring means coupled to said blocking means for receiving saidalternating current component therefrom and constructed to convert saidalternating current component into a unipolarity signal representativeof the power in said alternating current component; a bias voltagesource connected to provide a direct bias voltage signal of a polarityopposite that of said power representing signal; comparing meansconnected to receive said bias voltage signal from said voltage sourceand to receive said power representing signal from said power measuringmeans, and constructedto provide an output signal with a polarity thesame as that of the larger of said bias and power signals; a unilateralconduction device connected to receive said output signal from saidcomparing means and to pass a signal representative of only said powersignal when said power signal is larger than said bias signal; and asignal utilization device connected to respond to the passage of saidpower signal through said device.

5. A detecting system, comprising: limiter-frequency discriminator meansadapted to receive an input signal having noise and informationcomponents and constructed to convert said input signal into atransformed signal having an alternating current component,representative of the noise component of said input signal, and asubstantially direct current component; blocking means connected to saidlimiter-frequency discriminator means for receiving said transformedsignal therefrom signal therefrom and adapted to remove saidsubstantially direct current component from said transformed signal andto pass said alternating current component; power measuring meanscoupled to said blocking means for receiving said alternating currentcomponent therefrom and constructed to convert said alternating currentcomponent into a unipolarity signal representative of the power in saidalternating current component; a bias voltage source connected toprovide a direct current bias voltage signal of a polarity opposite thatof said power representing signal; comparing means connected to receivesaid bias voltage signal from said voltage source and to receive saidpower representing signal from said power measuring means, andconstructed to provide an output signal with a polarity the same as thatof the larger of said bias and power signals; a unilateral conductiondevice connected to receive said output signal from said comparing meansand to pass a signal representative of only one of said bias and powersignals when said one signal is larger than the other; and a signalutilization device connected to respond to the passage of said onesignal through said device.

6. A detecting system, comprising: frequency discriminator means adaptedto receive an input signal having noise and information components andconstructed to convert said input signal into a transformed signalhaving an alternating current component, representative of the noisecomponent of said input signal, and a substantially direct currentcomponent representative of said information component; blocking meansconnected to said discriminator means for receiving said transformedsignal therefrom and adapted to remove said substantially direct currentcomponent from said transformed signal and to pass said alternatingcurrent component which is reduced an amount functionally related to theremoved information component; thermocouple means coupled to saidblocking means for receiving said alternating current componenttherefrom and constructed to convert said alternating current componentinto a signal representative of the power in said alternating currentcom ponent; a bias voltage source, and an adder circuit connected toreceive a bias voltage from said voltage source and to receive saidpower representing signal from said thermocouple means and constructedto provide an output signal representative of the larger of said biasand power signal levels.

7. A detecting system, comprising: non-linear frequency selective signaltransformation means adapted to receive an input signal having noise andinformation components and constructed to convert said input signal intoa transformed signal having an alternating current component,representative of the noise component of said input signal, and asubstantially direct current component representative of saidinformation component; blocking means connected to said transformationmeans for receiving said transformed signal therefrom and adapted toremove said substantially direct current component from said transformedsignal and to pass said alternating current component which is varied anamount functionally related to the removed information component; athermocouple coupled to said blocking means for receiving saidalternating current component therefrom and constructed to convert saidalternating current component into a signal representative of the powerin said alternating current component; a bias voltage source; andcomparing means connected to receive a bias voltage from said voltagesource and to receive said power representing signal from saidthermocouple and constructed to provide an output signal representativeof the larger of said bias and power signal levels.

8. A detecting system for an information signal component lying in apredetermined frequency band, comprising: a band-pass filter adapted toreceive an input signal having a noise component and said informationcomponent; a limiter-discriminator circuit, frequency selective tofrequencies lying within said band, and con nected to receive a filteredoutput signal from said filter and to convert said output signal into atransformed signal having an alternating current component,representative of the noise component of said input signal, and asubstantially direct current component representative of saidinformation component; blocking means connected to said discriminatorcircuit for receiving said transformed signal therefrom and adapted toremove said direct current component from said transformed signal and topass said alternating current component which is reduced an amountfunctionally related to the removed information component; powermeasuring means having a predetermined upper frequency response; afilter connecting said measuring means to said blocking means; saidpower measuring means being constructed to convert an alternatingcurrent component received from said blocking means into a signalrepresentative of the power in said alternating current component; abias voltage source; and comparing means connected to receive a biasvoltage from said voltage source and to receive said power representingsignal from said power measuring means and constructed to provide anoutput signal representative of the larger of said bias and power signallevels.

9. The system claimed in claim 8 wherein said power measuring meanscomprises a thermocouple having a re sponse time of appreciably lessthan one second.

10. A detecting system for an information signal component lying in apredetermined frequency band, comprising: a band-pass filter adapted toreceive an input signal having a noise component and said informationcomponent; a limiter-discriminator circuit, frequency selective tofrequencies lying within said band, and connected to receive a filteredoutput signal from said filter and to convert said output signal into atransformed signal having an alternating current component,representative of the noise component of said input signal, and asubstantially direct current component; capacitor means connected tosaid discriminator circuit for receiving said transformed signaltherefrom and adapted to remove said direct current component from saidtransformed signal and to pass said alternating current component; powermeasuring means having a predetermined upper frequency response; an R-Cfilter connecting said measuring means to said capacitor means andhaving an upper frequency response of generally the same order as thatof said measuring means; said power measuring means being constructed toconvert an alternating current component received from said capacitormeans into a signal representative of the power in said alternatingcurrent component; a bias voltage source; and comparing means connectedto receive a bias voltage from said voltage source and to receive saidpower representing signal from said power measuring means andconstructed to provide an output signal representative of only thelarger of said bias and power signal levels.

11. The system claimed in claim 10 wherein said power measuring meanscomprises an integration network.

12. A signal detecting system, comprising: a limiterdiscriminatorcircuit frequency selective to a predetermined frequency band andadapted to receive an input signal having noise and informationcomponents lying within said frequency band and constructed to convertsaid input signal into a transformed signal having an alternatingcurrent component, representative of the noise component of said inputsignal, and a substantially direct current component; a blockingcapacitor connected to said circuit for receiving said transformedsignal therefrom and adapted to remove said substantially direct currentcomponent from said transformed signal and to pass said alternatingcurrent component: a power measuring circuit coupled to said blockingcapacitor for receiving said alternating current component therefrom andconstructed to convert said alternating current component into aunipolarity signal representative of the power in said alternatingcurrent component; a bias voltage source connected to provide a directcurrent bias voltage signal of a polarity opposite that of said powerrepresenting signal; signal adding means connected to receive said biasvoltage signal from said voltage source and to receive said powerrepresenting signal from said power measuring circuit, and constructedto provide an output signal with a polarity the same as that of thelarger of said bias and power signals; a rectifier connected to receivesaid output signal from said adding means and to pass a signalrepresentative of only one of said bias and power signals when said onesignal has an absolute magnitude larger than the other; and a go-no gosignal utilization device connected to respond to the passage of saidone signal through said device.

13. A signal detecting system, comprising: a limiterdiscriminatorcircuit frequency selective to a predetermined frequency band andadapted to receive an input signal having noise and informationcomponents lying within said frequency band and constructed to convertsaid input signal into a transformed signal having an alternatingcurrent component, representative of the noise component of said inputsignal, and a substantially direct current component; a blockingcapacitor connected to said circuit for receiving said transformedsignal therefrom and adapted to remove said substantially direct currentcomponent from said transformed signal and to pass said alternatingcurrent component; a power measuring circuit coupled to said blockingcapacitor for receiving said alternating current component therefrom andconstructed to convert said alternating current com ponent into apositive direct current signal representative of the power in saidalternating current component; a bias voltage source connected toprovide a negative direct current bias voltage signal; signal addingmeans connected to receive said bias voltage signal from said powermeasuring circuit, and constructed to provide an output signal with apolarity the same as that of the larger of said bias and power signals;a rectifier having a negative terminal connected to said adding meansand adapted to receive said output signal from said adding means and topass a signal only when said bias signal has an absolute magnitudelarger than said power representing signal; and a go-no go signalutilization device connected to respond to the passage of said onesignal through said device.

14. The system'claimed in claim 13 wherein said power measuring circuitis a thermocouple having a response time of appreciably less than onesecond.

15. The system claimed in claim 13 wherein said power measuring circuitis an integrating network.

16. In a signal detecting system of the kind adapted to sense thepresence of a substantially unifrequency information signal in a signalenvironment having a frequency band broader than that of saidinformation signal; the combination of frequency selective .signaltransformaiton means responsive to the frequency of said informationsignal and to at least a portion of said frequency band adjacentthereto; a signal power measuring device; said transformation meansbeing connected to provide an alternating current signal having apredetermined power content in the absence of said information signalthereto and a power content less than said predetermined power contentin the presence of said information signal; and said measuring devicebeing connected to provide an output signal representative of the powercontained in the alternating current output of said transformationmeans. 4

17. In a signal detecting system of the kind adapted to detect thepresence of a substantially unifrequency information signal in a signalenvironment having a frequency band substantially including thefrequency of said unifrequency signal; the combination of adiscriminator circuit responsive to the frequency of said informationsignal and to at least a portion of said frequency band adjacentthereto; a signal power measuring device; and a comparing circuit; saiddiscriminator circuit being connected to provide an alternating currentsignal having a predetermined power content in the absense of saidinformation signal and a power content less than said predeterminedpower content in the presence of said information signal; said measuringdevice being connected to provide an output signal representative of thepower contained in the alternating current output of said discriminatorcircuit; and said comparing circuit being connected to provide an outputindicative of the relative power contents of said information signal andsaid signal environment.

18. In a signal detecting system of the type adapted to sense thepresence of a substantially unifrequency information signal in a noiseenvironment having a frequency band including the frequency of saidinformation signal; the combination of a limiter-discriminator circuitfrequency selective to said frequency and to at least a portion of saidfrequency band adjacent thereto, a signal power measuring device, and acomparing circuit; said limiter-discriminator circuit being connected toprovide an alternating current signal having a predetermined powercontent in the absence of said information signal thereto, saidmeasuring device being connected to provide an output signalrepresentative of the power contained in the alternating current outputof said limiterdiscriminator circuit; and said comparing circuit beingconnected to compare said power representing signal with a predeterminedbias level and provide an output signal representative of the presenceof said information signal.

References Cited in the file of this patent UNITED STATES PATENTS1,663,086 Long Mar. 20, 1928 2,163,747 Crosby June 27, 1939 2,805,394Hermach Sept. 3, 1957 2,819,400 Toth Jan. 7, 1958 UNITED STATES PATENTOFFICE CERTIFICATION OF CORECTION Patent No, 2,970,304

January 51 1961 Emory Lakatos It is hereby certified that error appearsin the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below Column 7, line 55,

after direct,"

current insert column 8 line 2, strike out "signal therefrom"; columnll, line 10, for "absense" read absence (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of PatentsUNITED STATES PATENT OFFICE CERTIFICATION OF CORECTION Patent No.2370,1304 Januaryfil, 1961 Emory Lakacos It is hereby certified thaterror eppears in the above numbered p ent requiring correction and thatfohe said Letters Patent should read corrected below.

Column 7, line 55, after "direct" insert current column 8 line \2,strike out "signal therefrom"; column 1 line 10, for "absense" readabsence Signed and sealed this 22nd day of August 1961.,

( SEA L) Attest:

ERNEST W. SWIDER Commissioner of Patel UNITED STATES PATENT OFFICECERTIFICATION OF CORRECTION Patent No. 2370, 304 J's-xnlualryfil, 1961Emory Lakatos I It is hereby certified that error eppears in the abovenumbered pateht requiring correction and that the said Letters Patentshould read as corrected below Column 7, line 55, after "direct" insertcurrent column 8 line 2, "strike out ."signal therefrom"; column 11,line 10, for --"absense-" read absence sighed and sealed this 22nd dayof August 1961.,

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

